1. Field of the Invention
The present invention relates to a replaceable screen assembly for a vibrating shaker having a crowned deck. In particular, the present invention is directed to a screen assembly having a rigid frame and a top planar screening surface for use with a crowned deck vibrating shaker.
2. Prior Art
Various types of designs and configurations of vibrating screen machines have been utilized in the past. These vibrating shakers are used as screening and separation devices in various industries, such as in the drilling and mining industries to recycle drilling mud. A drilling fluid or mud is circulated from the surface, down through a drill string and down to a drill bit. After use in the well bore, the drilling mud, along with debris and drill cuttings, is brought to the surface where it is screened to remove solids over a certain size.
A screen or screen assembly is detachably secured to the vibrating shaker machine. With the screen assembly or multiple screen assemblies secured in place, a tray is formed with the opposed, parallel sidewalls of the shaker. The drilling mud, along with drill cuttings and debris, is deposited on the top of the screen assembly at one side. The screen assembly is vibrated at a high frequency or oscillation by a motor or motors for the purpose of screening or separating materials placed on the screen and fed thereover. The liquid and fine particles will pass through the screen assembly by force of gravity and be recovered underneath. The solid particles above a certain size migrate and vibrate across the screen or screens where they are removed.
It is known that to obtain the proper vibration of the screen assembly, slackness in the screen cloth or cloths must be discouraged. Any slackness in the screen cloth provides an undesirable flapping action of the screen which diminishes the proper vibration and also results in increased wear of the screen cloth. Accordingly, it is known that the screen assembly should be securely and tightly held down to the vibrating machinery. At the same time, the screen assemblies are subject to stresses from the vibrating machinery and wear over time and require periodic replacement.
One type of attachment mechanism includes hooks on each longitudinal end of the screen assembly to connect to the vibratory shaker machine. The shaker will have a channel-shaped drawbar on each side which mates with a corresponding hook on the screen assembly. The drawbars are held in place by bolts or other fasteners. These are detachably connected so that the screens may be replaced from time to time.
The vibrating shaker may include a bed or deck composed of a plurality of parallel cushioned rails on which the screen assembly or screen assemblies rest. Rather than having the bed or deck for the screen assembly flat or horizontal, the bed may be modified to be arched, bowed or curved upward so that the screen cloth or screen assembly is stretched tightly over the arched or curved surface. The height of curvature of the deck may vary from one half to one inch (1/2" to 1") from the center to the sides.
An early example of a crowned bed shaker is shown in the 1932 Flint U.S. Pat. No. 1,886,173. With a crowned bed, the screen cloth or screen assembly must be flexible enough to conform to the arch in the deck. Additional features to maintain tautness include spring loaded tensioning bolts to prevent loosening as the screens or screen assemblies stretch and seat onto the deck.
The crowned deck and accompanying crowned screen assembly can cause uneven fluid coverage. Because of the crowned deck, the fluid and solids deposited on the screen assembly to be separated will first gather at the sides. Depending on the fluid level, the arched center of the screen assembly may be exposed. The drilling mud to be screened may extend further out along the sides of the shaker deck than at the center where maximum deck height occurs. This will reduce the effective screening area of the vibrating shaker and reduce the efficiency. This condition can also lead to mud losses at the discharge and contribute to unacceptable wet cuttings if the drilling fluid passes across the surface of the screen assembly without being screened.
Several measures have been employed in response thereto. Manufacturers have modified the vibrating shaker to vary and alter the pitch of the deck itself so the drilling fluid is moving uphill from its entry to discharge. For example, the bed or deck may be set at an incline angle of from 1.degree. to 4.degree.. In other words, the fluid moves across the screen assembly and moves uphill at the same time. The higher the deck angle, however, the lower the screen life since solids will move slower across the screen, abrading as they travel across the screen.
The crowned deck will result in the fluid to be screened forming a U-shape toward the discharge end of the shaker.
An alternate measure employed is to corrugate the screen assembly to provide ridges to contact the fluid and assist in channelling the fluid.
Accordingly, it is known that flat panel screens can run at a lower deck angle than arched decks but do not have the advantages of the curved or arched deck in promoting tensioning of the screen assembly.
It is, therefore, a principal object and purpose of the present invention to provide a screening assembly having a flat, top planar surface for screening of fluids and solids which will mate with a crowned deck vibrating shaker.
It is a further object and purpose of the present invention to provide a process to convert a crowned deck vibrating shaker to a flat panel vibrating shaker.
It is a further object and purpose of the present invention to provide a flat panel screen for a crowned vibrating shaker which will increase the fluid throughput and, thus, the efficiency of the vibrating shaker.
It is a further object and purpose of the present invention to provide a process to convert a hook screen vibrating shaker to a non-hook screen vibrating shaker.